www.malwarebytes.com for any and all things spyware. Free. I Install it on every house call computer I have.
mayer said it, make sure the hard drive has AT LEAST 10% free space, the more the better.
schedule disk cleanup and disk defragmenter to run frequently at off hours.
RAM. Can't have enough of it. Too cost effective to not consider. At least 2 Gb. I always used to see how many windows of office docs, system tray apps and web browser windows a person has open. The user can usually be the culprit to a lot of "its sooooo slow" problems.
Restart the computer in the morning before starting to use it.
Did I say RAM?